The present invention relates generally to unplasticized polyvinyl chloride compositions, and more specifically to an unplasticized polyvinyl chloride composition for external use having improved resistance to ultraviolet degradation. 2. Brief Description of the Prior Art
Polyvinyl chloride is among the most widely used of synthetic organic polymer materials. Plasticized polyvinyl chloride compositions are widely encountered as, for example, "vinyl" sheet goods and as objects formed from plastisols. Polyvinyl chloride is commercially available in a variety of grades, some of which are suitable for preparing rigid, plasticizer-free compositions for extrusion. Polyvinyl chloride is subject to thermal degradation by dehydrochlorination. Since many processes for forming useful objects from polyvinyl chloride compositions, such as extrusion and molding, subject the composition to elevated temperatures, most include thermal stabilizing agents which tend to inhibit the thermal degradation of the polymer during processing. Examples of commonly employed thermal stabilization agents include barium/cadmium, and organotins including alkyl mercaptides, maleates and carboxylates.
Polyvinyl chloride is also subject to degradation by exposure to ultraviolet light. Articles formed from polyvinyl chloride compositions which are exposed to ultraviolet light, such as "vinyl" siding, and vinyl window and window frame components ("profiles") typically include an ultraviolet stabilizer. Among the most widely used ultraviolet stabilizers is titanium dioxide pigment. Filling a polyvinyl chloride composition with this pigment substantially reduces the effective depth of penetration of ultraviolet light into the surface of an article formed from such a composition. While rutile titanium dioxide is highly reflective at visible wavelenghts, it is also highly absorptive at ultraviolet wavelengths. However, although titanium dioxide is a highly effective ultraviolet light stabilizer for polyvinyl chloride compositions, it does have several serious drawbacks. An important disadvantage is the cost of titanium dioxide which has historically tended to be high compared with, for example, filler or extender pigments such as calcium carbonate, talc, and the like. Another significant disadvantage of using titanium dioxide as an ultraviolet stabilizer in unplasticized polyvinyl chloride compositions is that historically titanium dioxide has been periodically in short supply.
The relatively high cost of titanium dioxide is an especially significant disadvantage for the manufacture of articles for exterior use from unplasticized polyvinyl chloride compositions because such articles must often have substantially greater dimensions, for structural reasons, than the effective penetration depth of ultraviolet light in the articles. For example, profiles for constructing windows often have highly complex cross-sectional shapes, and multiple surfaces which are not exposed to ambient exterior light (and ultraviolet radiation) when the windows have been assembled and installed. Because the titanium dioxide is typically randomly distributed through the article, the bulk of the titanium dioxide is not located near the external surface, and thus plays no role in resisting ultraviolet degradation. Decreasing the level of titanium dioxide in the composition will tend to increase the effective penetration depth of ultraviolet light, and will consequently accelerate the degradation of the polyvinyl chloride and reduce the service life of the article.
It would be highly desirable to be able to reduce the level of titanium dioxide in such a composition without experiencing a concomitant increase in the rate of degradation and reduction in service life.